WASHINGTON — Republicans’ expansion of their Senate majority means that the conservative legal movement is poised to expand and entrench its influence over the federal courts, leaving Democrats with dwindling hopes of being able to swiftly diminish that imprint even if they win the White House in 2020.

The net pickup by Republicans of about three seats leaves President Trump and the Senate majority leader, Mitch McConnell, positioned to accelerate their already record-breaking pace of appointing federal judges through at least 2020. The White House and Mr. McConnell will now have more running room to get young and outspoken conservatives through even if a few Republicans break ranks, and they can consolidate the new conservative majority on the Supreme Court if a justice leaves during the next two years.

But the midterm results have longer-term implications as well. By broadening their Senate majority from 51 seats to about 54 (some races have yet to be decided), Republicans have expanded their chances of retaining control of the chamber after 2020. Only a handful of Republican Senate incumbents will be up in states where Democrats are competitive, and they have their own vulnerable incumbents, too.

As a result, even if a Democrat defeats Mr. Trump and takes over the White House in 2021, Mr. McConnell is likely to retain sufficient power in the Senate to prevent that president from appointing judges to start swinging the pendulum back. That situation would be similar to what happened in President Barack Obama’s final two years in office, when Republicans, after taking control of the Senate, systematically blocked his nominees to fill vacancies on both federal appeals courts and the Supreme Court.

“Getting judicial nominees acceptable from a liberal persuasion into a position to get nominated and confirmed is a slim-to-none proposition over the course of the next two to three election cycles because of the composition of the Senate,” said Bruce Buchanan, a political-science professor at the University of Texas, Austin.

The conservative legal movement, associated with networks like the Federalist Society, emerged in the 1980s in response to decades of liberal judicial rulings, and it has asserted increasing control over Republican judicial nominations. It has reached a new peak of influence under Mr. Trump, who vowed to appoint strongly conservative judges to shore up conservative support for his unconventional campaign in 2016.

In office, Mr. Trump has taken advantage of the abolition of previous Senate rules and practices that gave senators from the other party, even in the minority, some ability to constrain judicial appointments. In addition to appointing two young and strongly conservative Supreme Court justices, he has already appointed 29 appeals court judges. More nominations are pending, and the Senate could confirm them by year’s end.

By contrast, throughout Mr. Obama’s eight years in office, he managed to appoint just 48 judges to the regional courts of appeal. Senate Republicans blocked many of his choices, even in the minority, and then essentially shut down the confirmation process starting in 2015. That hardball strategy left numerous vacancies for Mr. Trump to fill.

“From the narrow perspective of judicial confirmations, yesterday’s elections were a huge success,” Ed Whelan, a conservative commentator on judicial confirmation issues, wrote on the National Review website. “Simply put, it ought to be an easy matter to confirm another outstanding judicial conservative (or two) to the Court. That increased margin, of course, also will make it easier to confirm lower-court judges. That in turn might lead to additional vacancies, as some retirement-eligible judges might decide that now is a good time to take senior status.”

While conservatives like Mr. Whelan laud Mr. Trump’s nominees as likely to rule according to the text and original meaning of the Constitution, liberals like Nan Aron, president of the Alliance for Justice, say that the Trump administration has been packing the courts with “right-wing extremists” who are hostile to reproductive rights, gay rights and civil rights, and excessively deferential to corporate power.

“The fear is that they will be able to cement their hold over the judiciary,” she said.

After the midterm vote, Mr. McConnell will have some breathing room from the razor-thin margins he has been working with — Justice Brett M. Kavanaugh was confirmed 50 to 48, with one Republican, Senator Lisa Murkowski of Alaska, defecting. The problem was also illustrated by the White House’s withdrawal in July of an appeals court nominee, Ryan W. Bounds, after Senator Tim Scott, Republican of South Carolina, objected to him over racially charged writings. Without Mr. Scott’s support, Mr. McConnell lacked sufficient votes to confirm Mr. Bounds.

Republicans increased their margin in the Senate by defeating Democratic senators in conservative-leaning states who had voted against confirming Justice Kavanaugh, like Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota, Claire McCaskill of Missouri and, apparently, Bill Nelson of Florida, although he has asked for a recount.

By contrast, a rare Democrat who survived re-election in a state that Mr. Trump won in 2016, Senator Joe Manchin of West Virginia, had broken with his party to vote for Justice Kavanaugh.

In 2020, Republican incumbents will be running in states where Democrats have a shot, including Colorado, Maine and North Carolina. But Democrats will have vulnerable incumbents up for re-election, too — especially Senator Doug Jones, Democrat of Alabama, who won a special election in the deeply conservative state last year against an opponent who had been accused of a pattern of sexual misconduct against minors.

Donald F. McGahn II, Mr. Trump’s former White House counsel and the chief architect of his judicial nominations until he left the administration last month, said that the red-state Democrats who voted against Justice Kavanaugh had “paid a price,” cementing the judicial strategy Mr. Trump and Republicans have followed.

“President Trump ran on judges. He has delivered on judges. And last night vindicates what he has done and shows that it will continue to happen,” he said. “And the overwhelming night the Republicans had in the Senate is going to pay dividends on judges even beyond this election. When one looks at the election map in 2020, it’s easy to see that Republicans are in an excellent position to maintain Senate control regardless of what happens elsewhere on the ticket.”

A version of this article appears in print on , on Page F4 of the New York edition with the headline: Lasting Implications for the Courts as Republicans Gain in the Senate. Order Reprints | Today’s Paper | Subscribe